The derivation of acoustically evoked electrical brain potentials of a subject is a known audiometric diagnostic method for testing healing and for evaluating various causes of hearing damage without the active participation of the subject.
This method is referred to in the field as ERA (electric response audiometry) or BERA (brainstem electric response audiometry) or brainstem audiometry. Areas of application for this method include for example the performance of the first hearing tests in newborns, testing the hearing of infants or of unconscious persons such as accident victims for example, and the diagnosis of neurologic diseases, for example nearinomas of the acoustic nerve. Intraoperative hearing tests are also possible with this method.
Electrical brain potentials are triggered by acoustic stimulation of the ear with conduction through air or bone. Headphones are usually used for the purpose. The electrical signals that are thus generated by the brainstem are picked up by electrodes applied to the head. Usually three electrodes are used, namely one electrode to determine the reference potential and two active electrodes to derive the acoustically-evoked electrical signals at two different locations on the head.
Acoustic stimulation of the ear can take, for example, the form of click stimuli or, for direct determination of the hearing threshold, of a rapid sequence of clicks with increasing volume. Other types of stimuli are of course also possible. The brainstem generates potential waves at each click which are averaged after being picked up and conducted away by the electrodes.
Previously, the electrodes were usually glued or attached mechanically in some other fashion individually to the head. Firstly, this is time-consuming and secondly, it imposes a stress on the patient. The cable connection also poses a risk of improper connection. In addition, the system is subjected to electrical stray fields; this is critical because of the extremely low signal potentials that must be detected. In addition, gluing the electrodes to the sensitive skin of newborns or infants and their subsequent removal is also a problem.
The combination of several electrodes into a single relatively rigid arrangement that can be mounted as a whole on the head of a person is known from U.S. Pat. No. 4,706,679 for the purpose of electroencephalography. In that patent, a frame is provided with a plurality of spring-loaded legs that have electrodes at their ends, and serves to pick up electrical brain potentials. In the known system, the frame is designed especially for mounting on the back of a patient chair in which the patient lies for the electroencephalogram. However, this prior art provides no suggestion for performing brainstem audiometry using electrodes combined in such fashion with an acoustic stimulator and a signal generator that produces the stimulating signal, or for deriving and evaluating the evoked brainstem signs.
From U.S. Pat. No. 5,954,667 a system is known, which allows a faster and simpler detection as a result of reuse of the electrodes forming part of the system and as a result of the fact that the electrodes are maintained in mutually fixed positions. Due to the extending arms the electrodes may however still present some difficulties during operation. The electrodes may moreover be difficult to clean, as they are located on a number of arms extending from a frame.
It is therefore an object of the invention to provide a device that permit simpler and easier mounting on the head of the individual subject.